The upgrade of modern communication systems makes the rapid development of the RF frond-end circuits. Thus the RF components should meet more requirements, such as high performance, small dimensions and low cost.
A balun is an essential component for the conversion between the balanced signals and unbalanced signals in many RF circuits, such as balanced mixers and amplifiers. In many applications, a filter is connected with the balun for signal selection, which increases the cost, size and complexity of the communication system. Thus, it is very necessary to integrate a balun and a filter in a single circuit. In recent years, lots of methods have been proposed to design balun filters. Firstly, a simple method is to integrate the balun and filter by using the internal matching circuit. However, the circuit topology is complicated and the circuit is bulky. Another method to implement the balun function is based on the bandpass filter which employs the unbalance characteristics between the input and output ports. The topology is relatively simple. However, it only can be achieved by using some specifical filter structures. Thus, this is not a common method. Besides, some symmetrical four-port networks can also be utilized to achieve the balun filter. In this invention, the balun filter using the coupled resonators is based on the phase characteristics of the resonator. The two filtering networks composed of half-wavelength resonators have balanced amplitude and unbalanced phased characteristic, which makes 180° phase difference at the two open ends. Thus, the balun filter is designed according to the theory.
In order to design the balun filter, various techniques have been employed, such as waveguides, cavities, and printed circuit boards. Although the good performance can be obtained, the complicated circuit structures lead to bulky size of the RF components, which is not suitable for practical applications.